A spark plug which is used in an internal combustion engine of an automobile and the like has a precious metal tip fixed on the tip for the purpose of improving the durability of the center electrode. A particularly useful material for the precious metal tip is iridium or an alloy thereof. For instance, Patent Document 1 describes a precious metal tip made from iridium, and Patent Document 2 describes a precious metal tip made from an alloy of iridium and nickel.
Patent Document 1: Japanese Patent Laid-Open No. 5-054955
Patent Document 2: Japanese Patent Laid-Open No. 1-319284
The precious metal tip is used for the purpose of improving durability, but a material which composes the tip is required to have excellent spark wear resistance, oxidation wear resistance and chemical resistance. This is because a spark plug is a component for generating a spark and thereby causing combustion in an internal combustion engine, accordingly is exposed to impact caused by the spark or to a high-temperature and highly oxidative atmosphere, and further contacts chemicals such as fuel or an oil additive.